1. Field of the Invention.
This invention relates coated surfaces upon which insects can neither climb nor alight and methods and means for applying the coatings. More particularly, the invention relates to surfaces coated with non-toxic, non-debilitating fluorocarbon resin films which are effective to discourage crawling insects from infesting a site where they are not wanted, to cause crawling insects to abandon a site they have infested, to confine crawling insects within a given enclosure and to prevent insects from alighting, climbing or building nests or webs on selected surfaces.
2. Discussion of the Prior Art
The crawling insects with which this invention is concerned are those which are pests or nuisances to man. These include, for example, gypsy moths that crawl from the ground up into trees and devour the leaves of the trees; cockroaches, spiders, scales and palm aphids which are often offensive in appearance and may carry and spread disease; and, perhaps the most widespread and annoying of the general group, the ubiquitous ant. The ant exists in many forms including the leaf eater, Argentine, carpenter and pharaoh ants, the termites and, of course, the militant and destructive fire ant. It is the primary concern of this invention to restrict the movement and colonization of these insects.
Many pesticides and insecticides are available which will kill or disrupt the life cycle of crawling insects, but essentially all of them are environmentally objectionable because of their toxicity. The toxicity of pesticides can endanger the health of animals and humans and curtail their use when food is present as in dinning rooms, kitchens, picnic tables, pet food dishes and the like. It is also well recognized that insects develop strains which are resistent to the effect of a given insecticide over a period of time.
As an alternative to insecticides, it has been proposed to control insects with traps. One common type of trap is built with downwardly and inwardly sloping side walls that are covered with a loosely adhering powdered material, such as talcum powder. The loosely adhering particles are dislodged when an insect attempts to crawl over them and the insect falls with the powder into a trap. This technique is relied on in U.S. Pat. No. 4,263,740 which teaches the application of particulate organic and inorganic materials to a vertical or sloping surface. The particulate materials are applied by coating a surface with a liquid dispersion of polytetrafluoroethylene, sand, clay, ground pyrophyllite-aluminum silicate, cellulosics, talc, sand, and flour. When the liquid content of the dispersion is evaporated, a dry, loosely adherent layer of powder remains on the surface. The size of the particles is not critical since patentee teaches that the particulate material of the dispersions may vary from less than 1 micron to more that 1000 microns in diameter, although his preferred range is from 100 to 300 microns.
In use, patentee relates, when an insect first crawls over a coated surface at the entrance to a trap, its sticky pads (pulvilli) are coated with the loosely adhered powders and the ability of the pulvilli to adhere to a smooth surface is rendered ineffective. If and when the insect reverses its direction in an attempt to crawl up a smooth slanting sidewall and escape from the trap, its pulvilli are no longer "sticky" and the insect must grip the particulate material with its tarsal claws to gain a purchase. At this point, the weight of the insect grasping the particulate matter dislodges the powder and causes an "avalanche" of the loosely held particles carrying the insect down with them into the trap.
The powdery coatings taught in U.S. Pat. No. 4,263,740 have a limited useful life. They are consumed in use when one or more "avalanche" occurs and they will break loose and flake away if the coated surface is impacted as by bumping or shaking or is exposed to the elements such as wind and rain. A less obvious defect in the powdery coatings is that loose powder can suffocate the insects and destroy their value to a hobbyist or an entomologist.
Entomologists have discovered that they can nondestructively confine crawling insects in tub shaped containers by painting circumferential, horizontal bands of a water dispersion of finely divided polytetrafluoroethylene, generally about 0.05 to 0.5 microns in diameter, on the inside walls of the container. Perhaps because of their extremely small size, the particles making up the dried coating, unlike the coatings of U.S. Pat. No. 4,263,740, adhere sufficiently well to the side walls of the container to prevent them from being pulled loose when traversed by an insect.
It was discovered that the insects could not climb these dried films of sub micron resin polytetrafluoroethylene particles. Since the insects appear to slide down the coated surfaces when they try to climb them, it seemed reasonable to assume, even though it has now been proven false, that the phenomenum was due to the slippery surface characteristics (low coefficient of friction) of polytetrafluoroethylene.
The dispersions that the entomologists have used are commercial formulations that typically contain about 60%, by total weight, of 0.05 to 0.5 micron sized polytetrafluoroethylene particles suspended in water along with from about 5% to 8%, by weight of polytetrafluoroethylene, of a wetting agent and stabilizer. At room temperatures (20.degree.-25.degree. C.) the dispersions have viscosities of from about 20 to 27 centipoise.
An example of the method used by entomologists, can be found in a trap for flying insects disclosed in European patent application 0 160 712 A1 to Montedison S.p.A. Here flying insects are prevented from crawling out of a trap having a funnel shaped (30.degree.-60.degree. cone angle) entrance by coating the interior surface of the funnel with dispersions of polytetrafluoroethylene. The dispersions are the same as those described above as used by entomologists which are commercially available formulations of aqueous dispersions of polytetrafluoroethylene. They are described in the patent application as being comprised of a granulometric dispersion of between 0.2 and 0.4 microns of 50-61% by weight polytetrafluoroethylene and 2.5-4% by weight of a non-ionic dispersant, the rest being water. It is further taught that the coatings of these dispersions can be applied by spraying or brushing. The European patent application acknowledges that the prior art has made use of sticky substances to entrap insects and powdered substances, such as kaolin or talc, to reduce an insects ability to cling to a surface. The patent application states that the powdered substances of the prior art are not useful over prolonged periods since the powders are easily removed by the mechanical action of wind, rain and humidity.
The coatings of the entomologists and those disclosed in the European patent application improved over the prior powdered coatings, but they too have their deficiencies both with respect to the methods by which they are applied and to the characteristics of the applied coatings. Considering the coatings themselves the following is a listing of some of their failings.
1. While the coatings adhere to their substrates somewhat better than do the powdered coatings, the polytetrafluoroethylene coatings are so loosely bound to their substrates that they can be removed by running tap water over the surface, by lightly brushing the surface, by flexing the substrate or by applying and removing ordinary transparent adhesive tape. PA1 2. The coatings are effective only for relatively short periods of time. In discussions with entomologists who use these coatings, it has been uniformly reported that the effective life of the coatings range from as short as several weeks to a maximum of about two, or, at most, three months. The mechanisms of the failures are not understood as often there is no apparent or observable change in the appearance of the coating. PA1 3. The effectiveness of the coating is adversely effected if more than a single layer is applied to a surface. Care is required to avoid overcoating when the dispersion is applied for, otherwise, multiple coating layers will crack or flake off rendering the coating ineffective to prevent insects from crawling on a surface. It follows that it is necessary to remove all of an old coating which has lost its effectiveness before a new coating is applied. The difficulty of thoroughly removing all of an old coating is enough of a chore to make it common practice among entomologists to discard used insect containers after they have lost their effectiveness rather than to clean and recoat the old containers. PA1 4. The effectiveness of the coatings are temperature sensitive. When exposed to ambient temperatures over about 30.degree.-35.degree. C., insects begin climbing over surfaces that they could not climb at lower ambient temperatures. PA1 5. Depending on the dispersing agent used in making up the polytetrafluoroethylene dispersions, most coatings loose their effectiveness when they become damp or wet. This limits the utility of the coatings to protected environments where they can be sheltered from the effects of rain, snow, dew and other surface moisture. PA1 6. The relatively poor adhesion of the coatings makes it impractical to clean the coated surfaces. For example, if a coating is applied to a dog dish, the coating will not survive even one dish washer cycle. PA1 1. The effectiveness of the coating is influenced by the thickness of the coating. It has now been discovered that better coatings are obtained if the thickness of the dried coating is preferably 1 mil or less. This desired thickness has not been obtained in the prior art for a number of reasons including the viscosity of the available polytetrafluoroethylene dispersions, the means by which the coatings are applied as by brushing or dipping, and the dependency of the thickness on the angle the substrate makes to the horizontal as it dries. In this latter regard it may be understood that if a freshly coated surface is allowed to dry while in a vertical position, the dispersion will flow by gravity down the plate, run-off will be maximized and the coating thickness will be minimized. If, on the other hand, the surface is oriented horizontally, the run-off will be minimized and the thickness of the coating maximized. PA1 2. The viscosity of commercially available polytetrafluoroethylene dispersions increases rapidly at higher temperatures. As a result, the coating thickness is a function of the ambient temperatures at the time the coating is applied. PA1 3. Polytetrafluoroethylene resins are soft and subject to fibrillation when exposed to shear forces. The effect is so pronounced that fibrillation occurs when polytetrafluoroethylene dispersions are forced through an orifice as in spraying. For this reason the dispersions are difficult to handle and, for practical reasons, cannot be sprayed unto a substrate. They can not be pumped through feed lines and care must be taken to avoid fibrillation if the dispersions are shaken, stirred or pumped. PA1 4. The high viscosity (about 20 centipoise and over) of the polytetrafluoroethylene dispersions of the prior art make it difficult to form a smooth coating because of the relatively high viscosity and poor flow-out. PA1 5. The polytetrafluoroethylene dispersions of the prior art are unstable and coagulate during prolonged storage. Temperatures above 25.degree. C. accelerate the coagulation of the dispersions and temperatures below the freezing point of the liquids destroy the dispersion.
The method of application of coatings from polytetrafluoroethylene dispersions as taught by the prior art, also raises a number of problems.